Liquid Crystal Displays (“LCDs”) are the technology of choice for avionics displays and a number of other applications. Among the advantages of LCDs are: they conserve weight and space, have simple electrical interfaces, are capable of preserving contrast for sunlight viewing with appropriate enhancements, can be made sufficiently rugged to withstand difficult mechanical and environmental conditions, can be made to operate over extreme temperatures and can be made compatible with night vision imaging systems. The LCD is a transmissive device; as such, it requires a source of rear illumination (backlight) to render the intended image. Each of the subassemblies of the overall system, e.g. LCD and backlight, shoulder different responsibilities in the integrated system. Some performance attributes are shared, while others are unique to each subassembly. As such, design considerations for each subassembly must be friendly to the other to ensure the integrated system can accommodate the rigor of the intended application.
Currently, the most popular method of back-illuminating an LCD is to use a serpentine fluorescent lamp in a reflecting housing with a heavy diffuser overlying the lamp to provide balanced luminance. The serpentine fluorescent lamp is mounted in a frame or housing having a reflecting surface, and a diffuser panel or sheet is then placed between the lamp and LCD. An alternative arrangement is the so-called “edge-lit” backlight subassembly which employs one or more thin, elongated fluorescent lamps each mounted along a side edge of a light guide. A sheet or film of reflective material is located on one side of the light guide, and one or more enhancement films such as light diffusers or Brightness Enhancement Films (BEF) are positioned on the opposite side of the light guide in between it and the LCD. When the fluorescent lamp(s) are illuminated, the light guide transmits and directs the light toward the enhancement films to illuminate the LCD.
Backlight subassemblies of the type described above are considered the “weak link” which can compromise the optical performance, environmental performance, and life expectancy of LCDs systems in many applications. This is principally due to shortcomings of the fluorescent lamps. Although an effective means of producing visible light, fluorescent lamps have a number of deficiencies which present difficulties when employed in backlighting subassemblies. Fluorescent lamps have poor reliability and poor efficiency when used in a backlight system. It is estimated that as much as 75% of the theoretical maximum luminance is lost with fluorescent lamp backlights. Fluorescent lamp backlights are not mechanically robust, they are difficult to dim, they are difficult to start at cold temperature and have significantly reduced light output at both high and low temperatures. The color gamut of fluorescent backlights is reduced as compared to LEDs, for example, and reduced color gamut of the light which illuminates the LCD reduces image fidelity. Serpentine fluorescent lamps have a relatively deep profile which increases the overall weight and size of the LCD system, making it less than desirable in some applications. Further, fluorescent lamps of all types contain mercury which presents a disposal issue when the lamps are replaced.
A color display is an additive color system. It takes at least one red, green and blue sub-pixel to make a white light color group. The white light color group is commonly referred to as a pixel. In an LCD, there are discreet color filters, e.g., red, green and blue, resident at each sub-pixel. These filters subtract unwanted wavelengths of light from the aggregate white-light backlight to produce the desired color. With the relatively recent (circa 1997) development of blue LEDs, considering that red and green LEDs were already in existence, the LED has been suggested as a replacement for fluorescent lamps in backlight systems as discussed, for example, in U.S. Pat. Nos. 5,727,862 and 6,719,436. The LED produces light when electrons flow across a P—N junction doped with the proper light-emitting compound. Whereas phosphor chemistry employed in the manufacture of fluorescent lamps is a mature science, LED chemistry is still in its infancy and significant gains in efficiency compared to fluorescent lamps, perhaps on the order of 200%, are expected in the coming years.
There are three competing configurations for generating white light using LEDs. These include an assembly which mounts discrete red, green and blue LEDs immediately adjacent to one another so that when they are collectively illuminated white light is produced, an ultraviolet LED coated with a red, green and blue phosphor coating, and, a blue LED coated with a yellow phosphor coating. To enhance the images produced by an LCD, the backlight spectra should produce emission peaks for the red, green and blue emission bands which match the peak transmission of the color filters on the LCD. The latter two approaches for producing white light noted above perform relatively poorly in that respect. Although improved performance can be obtained with an assembly which combines red, green and blue LEDs, the question has been how clusters or assemblies of such primary color LEDs can be efficiently and effectively incorporated into a backlight system to obtain the desired physical and other performance parameters, e.g. size, weight, durability, luminance, intensity, color gamut etc.